If your intended position for the bulkhead is only 1/4" off you're pretty lucky. At least you have something to work from. For many builders the bulkhead ends up between forms somewhere. Trace the form on cardboard and if you need more surface just add an 1/8" or whatever all around and get it to fit where you want it. The deck fit will never be perfect like the form since there is twisting and distortion as the boat halves get joined. What I do is put a line with a sharpe for position. I drill a small hole in the center and fasten the bulkhead to a stick so I can push it in position. I put a drop light in the front hatch and push the bulkhead in noting where it has to be shaved a bit. Pull it out, shave it and back in until I get it fitting perfect to the Sharpe line. This usually takes nothing more than a half hour or less. Than push it in with the glue or whatever. I do wood/glass bulkheads but that's the general technique I use to get the fit.

You can cut 3" foam with a hack saw blade holder, a jig saw, a filet knife is good, band saw and on and on. You will have to play with cutting or sanding the taper. But the foam has give so if the shape is decent, you can push it in with adhesive and it will squeeze into the shape.